The control of bleeding during surgery accounts for a major portion of the time involved in an operation. In particular, bleeding that occurs when tissue is incised or severed can obscure the surgeon's vision, prolong the operation, and adversely effect the precision of cutting. Blood loss from surgical cutting may require blood infusion, thereby increasing the risk of harm to the patient.
Hemostatic electrosurgical techniques are known for reducing bleeding from incised tissue prior to, during, and subsequent to incision. Bipolar electrosurgical techniques generally pass a high voltage-high frequency current through the patient's tissue between two electrodes for both cutting and coagulating tissue. This current causes joulean (ohmic) heating of the tissue as a function of the current density and the resistance of the tissue. The heat deposited in the tissue therefore coagulates the blood in the vessels contained in the tissue, thereby reducing the blood flow from severed vessels and capillaries.
Previously known electrosurgical instruments have generally conducted current to the patient's tissue in the form of a high voltage electric arc. For cutting tissue, the current magnitude and waveform may be selected so that the current arc causes evaporation of bodily fluids at a rate sufficient to sever the tissue. For causing hemostasis, the current arc provides a generally lower energy deposition rate that desiccates tissue to stem bleeding when the tissue is incised.
A drawback encountered with many previously known electrosurgical devices is that of controlling the current flow through the patient's tissue to obtain hemostasis in localized areas without also heating and causing undesirable trauma to adjacent tissue. Difficulty in predicting the depth of penetration of the electric arc creates uncertainty concerning precisely which tissue areas are being effected. Thus, for example, the electric arc may deposit insufficient energy to cause hemostasis at one site, while due to preferential resistance of the tissue, an electric arc of similar energy may lead to deep tissue necrosis if conducted to an adjacent tissue site.
Another drawback of previously known electrosurgical devices is the tendency of the current arc to promote charring of the tissue. In electrosurgical devices, the current arc and the patient's tissue form series components of an electrical circuit. The product of the voltage and the current represents the power loss attributable to each of these components. For previously known electrosurgical devices, the power dissipation in the current arc may exceed that in the patient's tissue. Consequently, the electric arc or flame generated by the electrosurgical device typically has very high temperatures, on the order of thousands of degrees. This electric flame can surround the tissue adjacent to the working surface of the device, and quickly lead to desiccation and charring of the tissue. While the electric flame thus cuts and causes hemostasis of the patient's tissue, it frequently results in charring of the tissue, which inhibits rapid regrowth of the tissue.
Yet another drawback of previously known electrosurgical devices, due in part to the wide variation in peak-to-peak voltage inducing the electric arc, is a tendency of the coagulated blood or severed tissue to adhere to the working surfaces of the instrument. This buildup, referred to as "coagulum," increases the electrical resistance of the path along which current flowing between the electrodes of the electrosurgical instrument must travel. A consequence of coagulum buildup on the instrument during an operation is that the electrical energy deposited in the tissue being heated or severed decreases, until the current flowing through the tissue is no longer sufficient to cause adequate cutting or hemostasis.
Consequently, the surgeon must frequently pause during surgery to scrape coagulum off of the working surfaces of the electrosurgical instrument. This scraping step increases the time and labor expended by the surgeon that is not directed to attaining the goal of the operation. Furthermore, inasmuch as this step of scraping the working surfaces of the instrument is not undertaken until there is inadequate hemostasis, there is additional blood loss from the severed tissue while the coagulum is scraped off the instrument.
A yet further drawback of previously known electrosurgical instruments is a tendency of tissue to adhere to the coagulum on the instrument. This sticking of tissue to the instrument can result in tearing of previously congealed tissue, thereby reactivating blood flow from that tissue. Additionally, such sticking of the instrument to previously congealed tissue can limit maneuverability of the instrument at the surgical site, thereby increasing the physical effort required by the surgeon to move the instrument about to accomplish the goal of the operation. Finally, such sticking, and the increased probability of reactivating blood flow by tearing previously coagulated tissue, can reduce the surgeon's field of vision of the working tip of the instrument and reduce the precision of the cutting.
Previously known electrosurgical instruments have employed generators generally providing alternating-current (AC) voltages in the range of 150 to 5000 volts peak-to-peak at power ratings of less than 400 watts. Such generators typically operate with current frequencies in the range above 100 kHz, because frequencies below 100 kHz are known to cause undesirable neuromuscular stimulation in the patient. It is also typical of previously known electrosurgical generators to provide power output to instruments rated between 100 and 400 ohms. To provide impedance matching of the power supply with the electrosurgical instruments, such power supplies also have high output impedance.
Malis et al. U.S. Pat. No. 4,590,934 describes an electrosurgical generator for use with a bipolar cutter/coagulator. The generator described in that patent generates a power output waveform comprising groups of aperiodic sequences of damped bursts of high frequency signals. The generator damps the high initial voltage spike at the onset of the electric arc generated by the electrosurgical device, to reduce sparking at the instrument tips and the undesirable equipment interference created by the initial spark of the electric arc.
Schneiderman U.S. Pat. No. 4,092,986 and Farin U.S. Pat. No. 4,969,885 describe generators for use with electrosurgical instruments whereby the output voltage of the generator is maintained at a substantially constant level, independent of the impedance encountered by the electrosurgical instrument
Schneiderman U.S. Pat. No. 4,092,986 describes the use of an unmodulated RF voltage waveform for cutting tissue and a pulse modulated RF voltage waveform for coagulating tissue. The patent teaches use of voltages in the range of 450 to 600 volts peak-to-peak with currents in the range of approximately zero to 0.6 amperes peak-to-peak.
Farin U.S. Pat. No. 4,969,885 notes that a minimum effective voltage of at least 150 volts (RMS) (420 volts peak-to-peak) is required for use in electrosurgical cutting instruments, in order to provide the electric field strength necessary to ignite and maintain electric arcs between the electrode and the tissue. That patent also notes that to provide constant voltage to the electrosurgical device throughout the anticipated range of operating conditions, it is desirable that the high frequency voltage generator provide a waveform that is independent of the operating conditions, and preferably a pure sine wave.
Electrosurgical instruments operated with voltages above 150 VRMS, and relatively low currents are believed to experience the coagulum buildup and associated problems described heretofore. These difficulties with coagulum buildup have limited the growth of the field of electrosurgery.
Herczog U.S. Pat. No. 4,232,676 describes an electrosurgical scalpel and method of use of that scalpel that attempts to overcome the drawbacks of coagulum buildup and charring associated with the use of high voltage electric arcs. That patent describes the use of low voltages, in the range of 20 to 80 volts, that prevent arcing and result in energy deposition rates of 5 to 50 watts. The scalpel described in that patent has heretofore achieved only limited commercial success, due in large part to the teaching of that patent that power be regulated by varying the frequency of the supplied voltage waveform.
It would therefore be desirable to provide an electrosurgical system that overcomes the problems of coagulum buildup and sticking that have plagued previously known electrosurgical devices and limited application of electrosurgery in surgical procedures.
It would be desirable to provide an electrosurgical generator capable of supplying low voltages at high power. Such a power supply would reduce arcing at the electrode, and the charring of tissue and sticking that typically accompany such arcing.
It would furthermore be desirable to provide an electrosurgical generator having a low output impedance that supplies a substantially constant voltage output level that is independent of the load impedance. Such a power supply would therefore maintain voltage at a preselected level, and thus avoid excessive energy deposition as tissue impedance increases during hemostasis.
In view of the limited room available in an operating room, and the size limitations imposed by heat dissipation requirements, it would also be desirable to provide an efficient and compact electrosurgical power supply.